In an engine-driven vehicle, a fuel tank housing fuel such as gasoline is provided. The fuel tank is configured by joining an upper tank and a lower tank by welding, and fuel is housed in a closed space formed by the upper tank and the lower tank. On a bottom surface of an inner part of the lower tank of the fuel tank, a sub-tank is generally provided, and it is configured such that even if the vehicle is inclined, a predetermined liquid level is constantly maintained to prevent a suction failure of fuel so that the fuel can be stably supplied to an engine. The sub-tank is fixed to the lower tank by spot welding in a state where a bottom surface of an outer part thereof faces the bottom surface of the inner part of the lower tank.
Regarding a fuel tank for a vehicle, due to a vertical vibration during traveling, a weight of fuel acts on a bottom surface of the fuel tank, and the fuel tank vibrates because it moves up and down, resulting in that a fatigue failure of welded portion at which a lower tank and a sub-tank are joined is caused, which is a problem. For this reason, a reinforcement is made by providing a bead on a bottom surface of the lower tank.
For example, Patent Literature 1 discloses a technique in which a sub-tank is attached to a bottom surface of a tank via a plate-shaped support to reduce a stress concentration on a welded portion, to thereby improve a flexural rigidity. Further, Patent Literature 2 discloses a fuel tank for a vehicle in which spot-welded portions at which a sub-tank and a lower tank are fixed are changed, and concave beads and a convex bead are linearly provided on a bottom surface of the lower tank. Further, Patent Literature 3 discloses a technique in which a stay is provided to make a reinforcement for preventing a separation between a sub-tank and a bottom surface of a tank. Further, Patent Literature 4 discloses a fuel tank for a vehicle provided with beads, on a bottom surface of a tank main body, which extend in different directions at a center portion in a longitudinal direction of the tank main body and at both side portions of an installation part of a sub-tank.